Fuel injector

ABSTRACT

A fuel injector for the direct injection of fuel into the combustion chamber of an internal combustion engine includes an actuator, which is encapsulated in a first sleeve and is sealed from an inner chamber of the fuel injector; a valve needle actuable by the actuator, which includes a valve-closure member cooperating with a valve-seat surface to form a sealing seat; and a locking spring, by which the valve needle is acted upon in such a manner that the valve-closure member is held in sealing contact at the valve-seat surface. An inflow-side end face of the first sleeve is dimensioned such that a linear deformation of the first sleeve, caused by the fuel pressure present at the inflow-side end face, is compensated by a linear deformation of the actuator and an activating piston extending the actuator.

FIELD OF THE INVENTION

The present invention is directed to a fuel injector.

BACKGROUND INFORMATION

A fuel injector is described in German Patent Application No. DE 195 34445. The fuel injector for internal combustion engines described thereinincludes a nozzle needle, which is axially movable in a nozzle body andcan be actuated by a piezoelectric actuator and which is held in aclosing position by a compression spring. The fuel is supplied by anexternal source at a freely adjustable pressure. The nozzle needle has acentral bore, and the actuator, which concentrically surrounds thenozzle needle, is sealed from the fuel pressure by a sealing surface.

A particular disadvantage of the fuel injector described in GermanPatent Application DE 195 34 445 is that, due to the influence of thefuel pressure, the lift of the piezoactuator can vary by up to 30% ofthe nominal lift, the sealing surfaces notwithstanding.

Furthermore, it is disadvantageous that, due to the length of the valveneedle, oscillations are induced therein, which can lead touncontrollable bouncing.

SUMMARY

An example embodiment of a fuel injector according to the presentinvention may have the advantage that the dependence of the valve lifton the fuel pressure is substantially reduced by pressure-compensatingmeasures. Compressive oscillations and actuator oscillations aredecoupled. This is made possible by a sleeve in which the actuator isencapsulated, the stiffness of the sleeve and the actuating piston beingselected such that a linear deformation of the sleeve is compensated bya corresponding linear deformation of the actuator and the actuatingpiston, which is disposed on the downstream side of the actuator.

It may be advantageous, in particular, if the actuator is prestressed ina simple manner by a spring, which is clamped between an actuatingpiston that is in operative connection with the actuator, and a sleevethat is joined to the housing of the fuel injector.

Furthermore, it may be advantageous if a prestressing of the actuator isaccomplished via a spring sleeve, which prestresses the two end faces ofthe actuator against each other via a support component and a shoulderof the actuating piston.

Due to the stationary arrangement of the second sleeve on the housing ofthe fuel injector, oscillations caused by the fuel pressure areeffectively damped.

The stiffness of the various materials of the bellows and the sleeve ofthe actuator are in this case selected in such a way that the lineardeformations are compensated.

In an advantageous manner, the first sleeve may also be encapsulated ina tube so as to avoid transverse forces, which make the longitudinalcompensation more difficult, and be sealed by a seal, so that only theend face is acted on by a force, but not the side surface of the sleeveencapsulating the actuator. This simplifies the compensation of thelinear deformation.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are shown in simplifiedform in the drawings and are elucidated in greater detail in thefollowing description.

FIG. 1 shows a schematic section through a first exemplary embodiment ofa fuel injector configured according to the present invention.

FIG. 2 shows a schematic section through a second exemplary embodimentof a fuel injector configured according to the present invention.

FIG. 3 shows a schematic section through a third exemplary embodiment ofa fuel injector configured according to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

A first exemplary embodiment of a fuel injector 1 according to thepresent invention, shown in FIG. 1, is configured in the form of a fuelinjector 1 for fuel-injection systems of mixture-compressing internalcombustion engines having externally supplied ignition. Fuel injector 1is particularly suited for the direct injection of fuel into acombustion chamber (not shown) of an internal combustion engine.

Fuel injector 1 encompasses a housing 2, which includes an hydraulicconnection 3 for the supply of fuel. Arranged inside housing 2 is afirst sleeve 4 in which an actuator 5 is encapsulated. In the firstexemplary embodiment, actuator 5 is designed as piezoelectric actuator5. On the inflow side, actuator 5 is braced on first sleeve 4 via asupport component 6. Also guided in first sleeve 4 is an electrical line7 for contacting actuator 5.

On the downstream side, actuator 5 is braced on an actuating piston 8,which penetrates a second sleeve 9 through an opening 10. Second sleeve9 seals first sleeve 4 from the fuel and is joined to housing 2 inform-locking manner via welded seams 11. Disposed between a shoulder 28of sleeve 9 and a shoulder 27 of actuating piston 8 is a spring 12,which prestresses actuator 5.

Formed in second sleeve 9 is at least one fuel channel 13, which allowsthe fuel to pass through to a valve group 14. Actuator 5 and actuatingpiston 8 are sealed from the fuel by means of a bellows 15 having anelastic design.

Formed on the downstream side of actuating piston 8 is a flange 16,which, in the closed state of fuel injector 1, is set apart fromactuating piston 8 by a gap 17. Flange 16 is in force-locking connectionwith a valve needle 19 via a welded seam 18. Valve needle 19 is guidedin a nozzle body 20, nozzle body 20 being connected to housing 2 by awelded seam 21. Disposed between flange 16 and nozzle body 20 is alocking spring 22. Locking spring 22 acts on flange 16, and thus valveneedle 19, with a closing force that keeps fuel injector 1 closed in thenon-energized state of actuator 5. In this way, a valve-closure member23, which is connected to valve needle 19, is held in sealing contact ata valve-seat surface 24 formed on nozzle body 20.

In response to an excitation of actuator 5, it expands in a dischargedirection of the fuel, counter to the force of spring 12, therebycausing actuating piston 8 to move in the discharge direction of thefuel as well. Gap 17 between actuating piston 8 and valve needle 19 isclosed. When actuator 5 expands further, valve needle 19 is movedcounter to the force of locking spring 22, in the discharge direction ofthe fuel, too. This causes valve-closure member 23 to lift off fromvalve-seat surface 24, and fuel is injected into the combustion chamber(not shown further) of the internal combustion engine.

Actuator 5 contracts when the electric current supplied via electricalline 7 to energize actuator 5 is switched off. In response to valveneedle 19 being relieved by actuating piston 8, locking spring 22 movesflange 16 counter to the flow direction of the fuel, so thatvalve-closure member 23, which is joined to valve needle 19, sets downon valve-seat surface 24 once again, thereby closing fuel injector 1.The force of spring 12 also moves actuating piston 8, counter to theflow direction of the fuel as well, so that actuating piston 8 returnsto its original position. Gap 17 is formed once more between actuatingpiston 8 and valve needle 19.

Due to the fuel pressure in an inner chamber 25 of fuel injector 1,first sleeve 4 in which actuator 5 is encapsulated is compressed. Inthis way, support component 6, which abuts against actuator 5 on theinflow side, is pushed in the discharge direction of the fuel, therebycompressing actuator 5 as well. Without compensating measures, thiswould have the result that gap 17, formed between actuating piston 8 andvalve needle 19, enlarges to an impermissible extent. Therefore, thestiffness of first sleeve 4 and the stiffness of actuating piston 8 aredesigned such that a linear deformation of first sleeve 4 in response tothe fuel pressure is compensated by a linear deformation of actuator 5and actuating piston 8, so that the totality of all linear deformationsadds up to zero and is thus unable to affect the lift of valve needle19. In this context, ΣΔ1=0, linear deformation Δ1 being proportional tothe acting force, so that Δ1=F/c. In this context, c denotes thestiffness of the acted upon material in response to the pressure force.Thus, in terms of the actual amount,F _(sleeve) /c _(sleeve) =F _(actuator/piston) /c _(actuator/piston).

On the condition that the diameter of fuel channel 13 is dimensionedsuch that the fuel flows in an unthrottled manner in the direction ofthe sealing seat, the pressure difference on the inflow side and on thedischarge side of second sleeve 9 is zero as well, so that, with F=p. A,it holdsF _(sleeve) /c _(sleeve) =F _(actuator/piston) /c _(actuator/piston).

Since c_(sleeve)>>c_(bellows), with the further restriction of theequation to the pressure relevant surfaces, the following resultsoverall:A _(sleeve) −A _(bellows) /c _(sleeve) =A _(bellows) /c_(actuator/piston).

The areas A_(sleeve) and A_(bellows) and the stiffnesses c_(sleeve) andc_(actuator/piston) may now be adapted in a simple manner by acorresponding form and material selection, in such a way that firstsleeve 4 encapsulating actuator 5 is pressure-compensated in connectionwith bellows 15.

Actuating piston 8 actuates valve needle 19 only when an electricvoltage is supplied to actuator 5 via electrical line 7. Thecompressional oscillations of the fuel and oscillations of actuator 5are decoupled by this measure, so that no undesired opening pulsesoccur. Therefore, it is possible to dispense with other, morecomplicated pressure-compensation technologies, such as an hydrauliccoupler. Furthermore, gap 17 is dimensioned such thattemperature-related linear deformations of the actuator cannot lead tomalfunctions of fuel injector 1.

In a part-sectional view, FIG. 2 shows a second exemplary embodiment ofa fuel injector 1 configured according to the present invention.Identical components have been given the same reference numerals. Adescription of already described components is not provided. Inparticular, valve group 14 may be identical to valve group 14 describedin FIG. 1.

The second exemplary embodiment shown in FIG. 2 is an especially simplevariant of fuel injector 1 according to the present invention. Actuator5 has an additional spring sleeve 26 in sleeve 4, which acts like atension spring. In this way, actuator 5 is prestressed and also providedwith an initial stress already prior to its actuation. An additionalspring, as in the first exemplary embodiment, may be omitted. In allother respects, the functioning method is identical to that of theexemplary embodiment described in FIG. 1.

In a schematic sectional view, FIG. 3 shows a third exemplary embodimentof a fuel injector 1 configured according to the present invention.Identical components have been provided with identical referencenumerals. A description of already described components is dispensedwith. In particular, valve group 14 may be identical to valve group 14described in FIG. 1.

In the exemplary embodiments shown in FIGS. 1 and 2, due to the force ofthe fuel that flows through fuel injector 1 and does not act exclusivelyon the inflow-side end face 29 of first sleeve 4, a lengthening of firstsleeve 4 by transverse forces cannot be avoided, this lengtheningcounteracting the compression of first sleeve 4 and making it moredifficult to compensate for the linear deformation.

In order circumvent this, the exemplary embodiment shown in FIG. 3proposes to encapsulate first sleeve 4 in a tube 30. While end face 29of first sleeve 4 still absorbs the force acting thereon, a side surface31 of first sleeve 4 is shielded, however, in that a seal 32 is insertedbetween tube 30 and support component 6. Seal 32 may be designed in theform of an O-ring, for example.

As a result, a chamber 33 that is free of forces is formed between firstsleeve 4, which in the present third exemplary embodiment is joined tosupport component 6 in a simple manner via a welded seam 34, and tube30, this chamber 33 preventing a lengthening of first sleeve 4, therebyfacilitating the compensation of linear deformations.

The present invention is not restricted to the exemplary embodimentsshown, but also applicable, for instance, to inwardly opening fuelinjectors 1 or magnetostrictive actuators 5.

1. A fuel injector for direct injection of fuel into a combustionchamber of an internal combustion engine, comprising: a first sleeve; anactuator encapsulated in the first sleeve and sealed from an innerchamber of the fuel injector; a valve needle actuable by the actuator,the valve needle including a valve-closure member cooperating with avalve-seat surface to form a sealing seat; a locking spring by which thevalve needle is acted upon in such a manner that the valve-closuremember, in a non-energized state of the actuator, is held in sealingcontact at the valve-seat surface; and an activating piston, whereinafter a gap between the activating piston and the valve needle isclosed, the actuator actuates the valve needle via the activatingpiston; wherein a stiffness of the first sleeve and a stiffness of theactivating piston is dimensioned such that a linear deformation of thefirst sleeve, caused by a fuel pressure applied on an inflow-side endface of the first sleeve, is compensated by a linear deformation of theactuator and of the activating piston, in such a way that an axialbreadth of the gap does not change in response to a change in the fuelpressure.
 2. The fuel, injector as recited in claim 1, furthercomprising: a support component which abuts against the first sleeve,the actuator being braced on the inflow side on the support component.3. The fuel injector as recited in claim 1, further comprising: a secondsleeve through which the activating piston penetrates via an opening;wherein the actuator is in operative connection with the activatingpiston.
 4. The fuel injector as recited in claim 3, wherein the secondsleeve is joined to the first sleeve and a housing of the fuel injectorvia welded seams.
 5. The fuel injector as recited in claim 3, furthercomprising: a bellows joined to the second sleeve, the bellowsconfigured to seal the activating piston from the inner chamber of thefuel injector.
 6. The fuel injector as recited in claim 5, wherein thestiffness of the first sleeve is substantially greater than a stiffnessof the bellows.
 7. The fuel injector as recited in claim 1, furthercomprising: a spring configured to apply an initial stress to theactuator.
 8. The fuel injector as recited in claim 7, wherein the springis disposed on a downstream side of the actuator.
 9. The fuel injectoras recited in claim 8 wherein the spring is clamped between a shoulderof the activating piston and a shoulder of a second sleeve.
 10. The fuelinjector as recited in claim 1, further comprising: a spring sleeve, theactuator being provided with an initial stress by the spring sleeve. 11.The fuel injector as recited in claim 10, wherein the spring sleeve isconnected by force locking to a shoulder of the activating piston on oneside and to a support component, which abuts against the first sleeveand on which the actuator is braced, on the other side.
 12. The fuelinjector as recited in claim 10, further comprising: a tube, the firstsleeve being encapsulated in the tube in such a way that a force exertedby the fuel acts only on the inflow-side end face of the first sleeve.13. The fuel injector as recited in claim 12, further comprising: aseal, the first sleeve being sealed from the inner chamber of the fuelinjector by the seal.
 14. The fuel injector as recited in claim 13,wherein the seal is an O-ring.
 15. The fuel injector as recited in claim12, wherein a chamber that is free of forces is between the tube and thefirst sleeve.